Growth and analysis of gallium phosphide on silicon for very high efficiency solar cells

Department: University of Delaware, Department of Electrical and Computer Engineering

Publisher: University of Delaware

Date Issued: 2011

Abstract: Photovoltaics research and development is marked by a continual effort to
reach higher efficiencies. The highest efficiencies are achievable through the use of
multijunction solar cells - devices designed to utilize the solar spectrum to its fullest
potential. State-of-the-art multijunction devices are comprised of III-V materials
both for the substrates and the junction layers. While these materials provide the
high efficiency sought, they are also very expensive. On the other hand, other
high efficiency devices utilize silicon (Si) - a material that is highly abundant and
inexpensive with a well-developed technological base. The goal of this body of
work is the integration of Si and III-V materials in a multijunction system for the
realization of high efficiency and increased affordability. This work is comprised of two portions. The first section is the experimental implementation of the growth of gallium phosphide (GaP) on Si through liquid phase epitaxy (LPE) based on previous foundational work. This approach provides unique challenges for the further growth of GaP or other III-V devices on a Si substrate. This body of work addresses the effects of substrate orientation, growth time, temperature, rate, and area, and principles of supersaturation. The second section is a theoretical analysis of two potential dual-junction devices: a GaP solar cell on a Si solar cell and a GaAsP solar cell on a Si solar cell. The results of this work show promise for the future of these devices. The experimental results of this study demonstrate marked improvements in GaP buffer layer quality for subsequent layer growths. Growth procedure optimization steps have led to a reduction of Si concentration from 20% to 3-10%. Additionally, theoretical modeling from a first principles approach shows relative efficiency gain of 20-71% and absolute efficiency gains of 3-13% for devices adding either a GaP junction or a GaAsP junction above a representative Si device. Both the experimental and the theoretical analysis shows that, while there is still work to be done to realize the goal of a high efficiency multijunction device utilizing a Si solar cell as the substrate, there is significant potential for these structures.